1. Field of the Invention
This invention generally relates to microlenses, and more specifically, the invention relates to injection molded microlenses that are particularly well suited for optical interconnects.
2. Background Art
There has been recent interest in the development of higher bandwidth fiber optic interconnects for a variety of server and storage applications. For example, optical transmitter arrays comprised of vertical cavity surface emitting lasers (VCSELs) are commercially available, with up to 12 lasers per array on a 250 micron pitch. These devices are interconnected with similar arrays of photodetectors using ribbons of optical fiber to form parallel optical interconnects (POI). These devices are available from a number of companies, including Agilent, Tyco, Emcore, Picolight, and Xanoptix. These devices may be used, for example, on high end technical computers as part of the clustering fabric between switches; this enables higher bandwidth, and longer distance links, which are essential to building large server, clusters. Smaller versions of POI, with only 4 optical elements per array, are also used in high volumes for I/O applications, and other uses are being developed for future clustering applications of this technology. Various widths of POI have been standardized, including 4×, 8×, and 12× arrays at data rates ranging from 2.5 Gbit/s/line to 5 Gbit/s/line. More advanced applications are also under consideration, including direct integration of VCSEL arrays into dual chip and multi-chip modules.
POI offers many technical advantages, including significantly higher bandwidths and many times the distance of copper links; immunity to electromagnetic interference; smaller, denser packaging; and lighter weight, more flexible cable assemblies. A significant inhibitor to the wider adoption of these links has been the relatively high cost compared with copper alternatives; consequently, POI is only used today in applications which are either insensitive to cost or which require a combination of distance and bandwidth that cannot be achieved any other way. Cost reductions for POI would thus be highly desirable.
A major cost component in POI is the active alignment required between an array of lasers and a corresponding array of optical fibers. It has been estimated that fabrication of such microlens elements is currently a $1 B market opportunity, and growing larger in the coming years. The goal is to launch infrared radiation (typical wavelengths near 850 nm) from the VCSEL aperture (initially 2-3 microns diameter) into the fiber core (typically 50 microns diameter) as efficiently as possible with the lowest cost. Positioning the fiber core directly against the laser aperture (butt coupling) is not practical because the laser beam from a VCSEL has very high divergence; it is not possible using standard optical array connectors (such as the MPO) to bring the fibers sufficiently close to the laser aperture. Even if this was possible and the beam diameter was smaller than the fiber core, losses would still occur due to a mismatch with the fiber's numerical aperture (part of the beam may still exceed the acceptance angle of the fiber and would not be guided).
An object of this invention is to enable wafer-scale manufacturing and electronic integration with optical components.
For this reason, all practical VCSEL arrays employ some form of lens structure to facilitate coupling light into the fiber array. This problem is significantly more complex than alignment of a single laser and fiber, due to effects such as cumulative tolerance runout in the lens and VCSEL designs. Conventional lens elements can be fabricated separately (for example, spherical or ball lenses made of glass), then manually aligned with elements in the VCSEL array; this is not a low cost manufacturing process, and uniformity of the coupled optical power is not well controlled across the array. There is also high manufacturing fallout from the failure to properly align lasers and lenses, or the failure of a laser array element after alignment, which is an important reason for the high costs encountered today. Thus, there is an industry need for a low cost, high volume fabrication method for VCSEL array lenses, and a low cost assembly/alignment procedure for attaching these lenses into a VCSEL package.